The present invention relates to a disposable, accurately rated thermometer probe and particularly relates to a simple method for accurately temperature-rating a thermometer probe for use in a temperature readout.
Various disposable thermometers have been developed. Disposability eliminates the cost of sterilizing thermometers for reuse. Disposable thermometers must be inexpensively and rapidly produced. Yet, they must be sufficiently accurate to satisfy recognized clinical standards, e.g. for readings of human body temperature, they must be accurate to within 2/10ths of a degree Fahrenheit over the range of human temperatures, i.e. around 98.6.degree. F. or 37.degree. C.
The inventor hereof has previously devised a thermometer system comprised of disposable thermometer probes and of a readout into which a probe is plugged for providing a temperature. See U.S. Pat. No. 4,317,367. The disposable probe utilizes a temperature-sensing element whose electrical properties vary proportionately with the temperature of the sensing element, and one temperature variable electric property of the sensing element is converted by the readout into a temperature reading.
In one preferred probe, a thermistor, which is a ceramic element with a negative temperature coefficient of resistance, serves as the sensing element. The thermistor is mounted on a supporting substrate, is sheathed in a thin, heat-transmitting sheath and is connected in circuit with conductors that extend along the length of the substrate. The substrate and the conductors thereon are plugged into a readout which senses the resistance of the circuit including the thermistor and converts the sensed resistance into an indication of the temperature of the thermistor.
During its manufacture, each thermometer probe is temperature rated against a standard. In the present inventor's previous method, the temperature-sensing element of the probe is adjusted, so that every probe will produce the same temperature indication in the readout when the sensing elements of the probes are all at the same temperature.
The inventor's previous method of rating thermistor probes is described in U.S. Pat. Nos. 4,200,970 and 4,236,298. An assembled thermometer probe is connected in circuit with apparatus that measures electric resistance, the resistance of the probe circuit is measured, and part of one of the contacts on the thermistor is removed to adjust the resistance of the probe to a predetermined value. Although the prior method is effective, the steps involved can cause that method to be somewhat more expensive than the method disclosed herein. In particular, in the prior method, the probe to be rated is first measured for its resistance. To then adjust its resistance, a portion of a contact of the thermistor is trimmed away. The probe resistance is again measured and, if further adjustment is needed, further contact trimming is performed, until the resistance of the probe is brought within an acceptable range of the desired resistance, corresponding to an acceptable deviation from precise temperature probe accuracy. Each separate trimming and resistance-measuring step require some time. Also, whatever thermistor contact trimming technique is used, the thermistor is shocked by impact, pressure and/or vibration and by the rapid temperature rise which the thermistor experiences during trimming. Too severe a temperature shock could permanently alter the thermistor's characteristics. In any event, thermistor contact trimming will at least for a short time, raise the temperature of the thermistor, and the rating of the respective probe cannot again be checked until the heated thermistor returns to its previous temperature level. Finally, all temperature probes used in the prior method must have slightly lower resistances than the desired resistance, since trimming away part of a thermistor contact will always raise the resistance toward the desired level. If too much thermistor contact material is removed during the adjustment process or the thermistor initially has too little contact material, the probe will have too high resistance, and it must be discarded, since there is no practical technique for again reducing the resistance to the desired level. Hence, frequent probe discards are created due to the manner in which the thermistor probes are rated. Since unusuable probes have to be discarded subsequent to their assembly, there is wasted effort and materials used in fabricating such probes.
Another well known rating techique is providing matched pairs of thermistors, comprising selecting, from a large supply of thermistors, two thermistor pieces which together have the desired resistivity. This uses double the number of thermistor pieces and time is wasted in searching for matched pieces.